This invention relates to optically measuring the following properties of suspended polvdisperse particulates: integrated volume concentration, integrated surface area concentration, and aerosol infrared extinction coefficient. These properties can be given the name particulate integrated properties.
Other optical techniques exist to measure the preceding integrated properties of particulates. These techniques can be categorized into two broad types: In the first type, a light beam is used to irradiate one particulate at a time, and the light scattered out of the beam by the particulate and measured by a light detector gives an indication of the desired property of the particulate. A typical example of this single-particulate optical technique is described by W. D. Bachalo, U.S. Pat. No. 4,854,705, Aug. 8, 1989. In order to determine the volume, area, and extinction particulate integrated properties listed in the preceding, such single-particulate optical techniques are required to sum up electronically the light-scattering contribution from many individual particulates.
Given the small size of typical particulates, which is often in the order of microns, and the resulting requirement for microminiature and highly precise measuring optics, it is difficult for the single-particulate optical techniques to achieve high accuracy in the measurement of the particulate integrated properties. The second broad type for optically measuring particulates minimizes this difficulty, because in the second type a light beam irradiates many particulates in the same time interval, and the light scattered by the many particulates is measured by a detector that can output a signal directly proportional to the particulate integrated properties. Examples of this multiple-particulate optical technique are given by W. L. Wilcock, U.S. Pat. No. 3,873,206, Mar. 25, 1975, and H. E. Gerber, U.S. Pat. No. 4,597,666, Jul. 1, 1986.
The previous multiple-particulate optical techniques use a collimated light beam to irradiate the particulates. The collimated-beam feature permits the use of simple detector geometries (for example, slit-like for the Wilcock patent, and circular detector for the Gerber patent) to determine directly particulate integrated properties; however, the collimated-beam feature leads to some limitations that the present invention attempts to overcome. In the earlier techniques the collimated beam and scattered light usually pass through a lens, on the other side of this lens the beam is focused into a light trap, and the scattered light continues on to a large-area detector placed just past the light trap. The beam passing through this lens scatters light from unavoidable imperfections in the glass of the lens, as well from possible contamination on the lens, surfaces; this scattered light interferes with the measurement of light scattered by the particulates, and prevents the use of this technique (with collimated beam passing through lens) for measuring particulate concentrations and mass loadings important for environmental monitoring. An obvious alternative is to place the light trap in front of the lens, so that the collimated beam is not required to pass through the lens. This alternative, however, requires that the collimated beam and light trap are very narrow, so that the required small light-scattering angles from the particulates are not blocked from the detector. The narrow beam of this alternative results in a small irradiated volume that under conditions of low particulate concentration is a disadvantage.
This patent application described an optical method and apparatus that utilizes a noncollimated convergent beam irradiating many particulates in the same time interval. The convergent beam falls into a light trap in front of the lens that collects the scattered light. In this manner no light from the beam is scattered by the lens; and the noncollimated beam can illuminate a much larger volume containing particulates than can the narrow collimated beam. This leads to an apparatus that is more sensitive to low particulate concentrations, and that can be more compactly constructed.
The use of a convergent noncollimated beam does not, however, lend itself to the simple and direct measurement of the particulate integrated properties obtained with the simple detector geometries used in the collimated-beam techniques. This invention describes a method, based on using an inversion spatial filter placed in front of the detector, by which the present noncollimated technique can make direct measurements of particulate integrated properties.
This invention further describes the use of a second detector assembly placed in such a way with respect to the concollimated beam to enhance the scattered light contribution from small particulates, which are underestimated in the single detector version. This permits a particulate size range to be measured (0.1 um and larger) for the particulate integrated properties that is consistent which required atmospheric monitoring.